1. Field of the Invention
The application relates to a photoacoustic detector that has a high level of selectivity for measuring gases.
2. Background Description
Different types of detectors/sensors and analyzers can be used to measure and monitor the concentration of gaseous, air-contaminating substances in the air or impurities in process gases. An important family of gas sensors uses the selective absorption of light by the gas molecules.
Because the absorption spectrum is a very characteristic property of a molecule, absorption spectra or well selected absorption lines can be used to differentiate different contaminants or impurities, which are normally present in a gas.
The absorption of light in a gas can be measured in various ways. One method uses the so-called photoacoustic effect, the production of sound through light absorption. It is known that a photoacoustic signal can be produced only if the absorbed light energy is not constant, but changes from time to time. As a result, only modulated or pulsed light can produce a photoacoustic signal.
In photoacoustics, the intensity of a light beam, normally a laser beam, is periodically modulated either through a rotating chopper blade or the electronic driver unit of a laser source or light source. A further possibility is the modulation of the absorption in that, for example, the wavelength of a laser beam is periodically adjusted by a sharp absorption line of the molecule, which is to be detected (wavelength modulation). In this case, the light output of the light source, for example of the laser, remains constant, but the absorbed portion relevant for the photoacoustic measurement changes periodically. A third possibility is the use of a periodic pulse sequence of a pulsed laser.
Photoacoustic gas sensors use almost exclusively lasers as light sources. Previously, line-adjustable gas lasers such as CO2 lasers or CO lasers were used, nowadays mainly diode lasers, quantum cascade lasers or adjustable non-linear laser sources are used. However, these devices are expensive. Although the photoacoustic detector is also simple and cheap per se, however, due to the price of the laser source, which accounts for between 80% and 90% of the total cost, these devices are suitable only for applications in which their outstanding capacity is also truly required.
Less expensive photoacoustic gas analyzers or gas monitoring devices use a broadband infrared source (incandescent lamp, infrared emitter, etc.). Because the light from these sources is absorbed by almost all molecules, it is very difficult to differentiate the species that are present in the air being monitored or in a gas being monitored. In order to improve the selectivity of these devices, in addition further spectrally selective elements must be used. Two examples of photoacoustic instruments that use broadband light sources and non-dispersive optical absorption are dealt with in detail in the two following sections.
A broadband light source is used in the case of gas analyzers made by INNOVA AirTech Instruments. In these commercially available instruments, a heated incandescent lamp is used as the light source. The incandescent lamp is localized in the first focus of an elliptical gold-coated mirror. A small photoacoustic cell is positioned such that the infrared light is focused in the center of the photoacoustic cell with the aid of the elliptical mirror through an infrared light-permeable window. The light is modulated by a rotating chopper wheel, and six different wavelength ranges can be selected by interference filters that are installed on a carousel wheel. This instrument has a sensitivity in the parts per million by volume (PPMV) range, however, its selectivity is limited because various other components possibly absorb in the wavelength range selected by the interference filter.
Another type of gas monitoring device (e.g., URAS made by ABB) uses a photoacoustic principle to detect the optical absorption. In this case, the light of a broadband infrared emitter is separated into two equal parts and modulated by a chopper. Then the two beams pass through two identical gas cells and reach two chambers of a differential detector. The two chambers are separated, but both are filled with the gas to be detected. Since the broadband light that reaches the chambers always includes the wavelengths that can be absorbed by the target molecules, photoacoustic signals are always produced in both chambers of the differential detector. If no light is absorbed in the gas cells on the way to the detector, the same light energy reaches both chambers, hence the photoacoustic signals in both chambers are the same and no differential signal appears as a result. In normal operation, however, the first gas cell (reference) is filled with a non-absorbing gas or gas mixture, while the second gas cell (sample) is filled with a gas to be monitored, for example, air. A portion of the light is absorbed in the sample cell, thus less light energy reaches the second chamber of the differential detector than the first chamber. For this reason, unequal photoacoustic signals are observed. The difference signal is greater if more light is absorbed in the sample cell. This instrument can be used as a monitoring device for the molecule that fills the differential detector. Its sensitivity is limited, however, because, in the case of a weak absorption in the sample cell, the difference signal in the detector is low. The limit of detection is thus determined by the offset of the infrared intensity and the fluctuations thereof on the two light paths.
A device is known from WO 2005/093390 A1 in which the radiation of a pulsed light source is alternately guided through a measuring cell A and a reference cell B into a photoacoustic measuring cell C. The absorption in the cell B is largely known, because a known gas with a known concentration is located there. Measuring cell A and reference cell B have an identical structure and differ only with regard to the gas contained. The difference of the photoacoustic signal, which is generated after passage of the radiation through cell A and cell B into the cell C, is a measure of the absorption in the cell A. Thus, inferences can be drawn about the type and concentration of an analyte in the cell A.
A similar structure is known from GB 2 358 245. This case also provides for radiation to be directed through a measuring cell and a reference cell and the quantity of the radiation transmitted to be determined photoacoustically in a further cell. As a result, the difference of the radiation directed through measuring cell and reference cell and thus the concentration of an analyte in the reference cell can be determined.